An isotactic polypropylene (iPP) sample (100% by FTIR) has been thermooxidized in the temperature range 80-170 degreesC and the kinetics of the degradation followed by measuring the chemiluminescence intensity along the induction and autoacceleration stages. From those curves, the parameter p(0), related to the initial concentration of oxidizable sites, has been calculated and compared to previous results on other PP of different tacticity. A relationship between the relaxations and/or phase transitions taking place in the temperature range under study and the temperature dependence of parameter p(0) in the four samples of fully or predominantly i-PP has been found. It is proposed that the discontinuities in the rate of release of interchain interactions caused by these phase transitions or relaxations are responsible for the slope of the temperature dependence of log p(0). Polymers bearing alpha-relaxation show a strong decrease in p(0) at temperatures under 115 degreesC which is absent in polypropylene samples where the a-relaxation is not resolved. As the a-relaxation has been attributed to the diffusion of chains from the amorphous to the crystalline phase and through the interphase, it is proposed that the a-relaxation makes it possible for crystals to act as radical scavengers and thus contribute to stabilize iPP at temperatures under 115 degreesC.